Radio receiver



23, 1945- H. c. LAWRENCE. JR

RADIO RECEIVER Filed April 24, 1943 mo/a mm? M L Zinnentor b ow/ma Gage- ZAWAM-Wcf u'R; r w

attorney volume control,

I control charac 'raeeieaoeezalus UNITED STATES r RADIO RECEIVER v Howard rey Lawrence, In,

alslgnor to Radio Corporation of America, a 7 corporation of Delaware Application April 24, 1943, serial No. 484,365 1 Claims. (Cl. 259-20) This invention relates to radio receivers and more particularly tonoise limiting and the preventionof overloading effects in continuous wave receivers which are no equipped with automatic volume control.

Receivers intended for use with radio beacons antenna, the received signal strength varies as a function of the alignment between the antenna and arriving wave front. If such an antenna is operated with a receiver provided with automatic the receiver tends to produce constant output regardless of variations strength in the received signal, so that as the antenna is rotated no sharp variation in receiver output will occur and no indication, or a very broad indication, of the direction of the wave front will be given.

However, some form of volume control is essential in substantially any radio receiver to limit noise and to prevent saturation or overloading when the receiver signal strength exceeds the capabilities of the receiver. This is particularly 30 beacon receivers and the an true of direction finde like, since excessive overloading may cause the receiver output to decrease upon further increase in si nal strength and increase upon decrease of signal strength, causing false bearing indications. It is the principal Object of the instant invention to provide an improved method of and means for preventing overloading effects and limiting noise in a radio receiver which is all". ranged for reception of keyed Other objects will become apparent to those skilled the art upon consideration of the following description with reference to the accompanying drawing. of which:

Figured is a schematic diagram of a portion 45 of the circuit of a radio receiver embo ying a limiter arranged in accordance with the inven- .tion,

Figure 2 is a modification of the system of Figure 1, and r Figure '3 is a 1 and 2.

Referring to Figure 1, a radio receiver l of the 9 A resistor i5 may be provided in serieswith the with its last intermediate frequency amplifier tube 3 connected to a tunedcircuit comprising a capacitor} and an inductor I. This circuit is tuned to the intermediate frequency of the receiver I and is coupled through a capacitor 9 to a detector. II. The output circuit of the detector II is coupled in known manner to the usual audio amplifier and loud speaker or the-like, not

shown. A gaseous discharge tube Il-is connect- ,ed across the tuned circuit elements 5 and 1.

tube IS in order to-modiiy the action of the limiter circuit, as explained more fully hereinafter. A transformer H is provided with a secondary winding 19 connected between the lower ends of the elements 5 and 'l and the return connection to the output circuit of the amplifier 3. The impedance of the winding l9 must be low compared 01 the 20 ignals, and

- amplitude of to the resonant impedance of elements 5 and l. Theprimary of the transformer I1 is connected to a beat oscillator 2|.

The operation of the above described system is as follows:

As long as the amplitude of the output of the amplifier 3 is lower than the predetermined value, the gas in the tube l3 remains in an un-ionized state so that no conduction takes place through the tube 13. The beat oscillator 2| is tuned to a frequency differing from the intermediate frequency of the receiver I by a relatively low amount; 'for example, L000 cycles. The outputs of the amplifier 3 and beat oscillator 2i are applied in series to the detector I I. Keyed continuous wave signals applied to the grid of amplifier I are converted by the detector II to audio signals of a frequency equal to the difference in frequency between the beat oscillator and the intermediate frequency. When the output of the receiver I exceeds the predetermined amplicuit I, I cannot increase above a level determined by the characteristics of the tube IS. The the output of the detector" II is a function of the amplitudes of the IF output voltage of the receiver l and the voltage produced by graphical representation of the ristics of the systems of Figures the oscillator 2]. Therefore, if the beatoscillator output has remained constant, the detector output will also remain 'at a constant level. It

shouldbe notedthat the beat oscillator voltage conventional superheterodyne type is arranged so is applied in series with rather than across the ATENT OFFICE. n

tube It. This is done in order to prevent the limiting action the tube it. from changing the output of the beat oscillator and removing the audio modulation established by the beat oscillator.

' 6 Referring to Figure 3, as the input voltage I from the amplifier 3 increases, the output voltage will increase correspondingly up to the point 23. At this point the gas in the tube becomes ionized and further increase in input will produce 10 little or no increase in output, as indicated by the solid line 2!. Under some circumstances, it may be desirable to allow a further increase in output to take place even after the point of ionization of the tube It is reached. This m y be accomplished by including the resistor Ii in the, circuit of Figure 1. Then, as the current through the tube It increases with increase in input from the IF amplifier I, the drop in the resistor It will increase proportionally. providing a resultant characteristic for both the gas tube the action of said discharge tube.

assmes I claim as my invention:

1. In a radio receiver, means for provi carrier-frequency signal voltage, a gaseous discharge tube, means for applying said signal voltage to said gaseous discharge tube 'whereby o'utput of said oscillator to said detector in series and resistor, as illustrated by the dash line 21.

Figure 2 shows a modified circuit employing inductive coupling between the plate circuit of the intermediate frequency amplifier and the detector. Portions of the circuit of Figure 2' which correspond to the circuit of Figure 1 are designated by similar reference numerals. The plate circuit of the IF amplifier tube 3 may be connected to a tap 29 at an intermediate point ,on the inductor I to provide step-up by autotr'ansfo'rmer action of the voltage applied to the tube iii. A secondary wihding 3| is inductively coupled to the inductor I and is arranged to be tuned to resonance by' a shunt capacitor. The detector ii comprises a diode connected in coning a gaseous discharge tube and a resistor conventional manner to a load resistor and coupled through a capacitor 31 toan audio frequency -amplifier, not shown. The load resistor 35 is shunted by a capacitor 39 to by-pass intermediate frequency voltage around the resistor 35. The

beat oscillator 2i is coupled through a transformer I! to the cathode circuit of the diode. i i. The operation of the system of Figure 2 is substantially the same as that of Figure 1.

Thus the invention has been described as an improved limiter for radio receivers intended for reception of keyed signals. A variable impedance having an amplitude limiting characteristic, such 5 as a gaseous discharge tube, is connected across the plate circuit of the last intermediate fre-' quency amplifier tube, and a beat frequency oscillatoris connected in series with said gas tube.

This arrangement allowsthe received signal and any noise which may arrive with it to be limited to a predetermined amplitude without removing the modulationproduced by the beat frequency oscillator.

relationship with the voltage across said nonlinear impedance device.

'3. In a radio receiver including means for deriving a continuous wave signal ,voltage, a detector, a voltage limiter connected across said means, an oscillator, and means for applying the output of said oscillator in series with that across said limiter to said detector.

4. In a radio receiver including means for deriving a continuous wave signal voltage, a nonlinear impedance device connected across said means, said non-linear impedance device includnected in series therewith, anoscillator, a detector, and means for applying the output of said oscillator to said detector in series with said continuous wave voltage.

5. A radio receiver including at least one intermediate frequency amplifier tube, a resonant circuit connected to the anode circuit of said amplifler tube, a gaseous discharge tube connected across said resonant circuit, a local oscillator, a

detector, and means for applying voltage appearing across said discharge tube andthe output of said oscillator in series with each other to said detector. Q I 6. A radio receiver system including an intermediate frequency amplifier tube, a resonant circuit connected to the anode of said amplifier tube, v a voltage limiter connected across said resonant circuit, a detector, a local oscillator, and means for applying voltage appearing across said limiter and the output of said oscillator in series with each other to said detector.

' '7. The invention as set forth in claim 1 wherein said means for applying said si nal voltage to I said gaseous discharge tube includes voltage stepup transformer means.

nowann com LAWRENCE, on. 

